1. Field of the Invention
This invention relates to magnetic levitation devices and, more particularly, to such devices in formats that utilize an axially-symetrical divergent magnetic field to produce a lifting force that supports the weight of a magnetically responsive levitated object. Such lifting forces can be generated by the attraction of two magnetic elements or by the repulsion of two magnetic elements. Some of the versions of this invention described herein pertain to attraction type levitation systems while others pertain to repulsion type levitation systems. Certain features of this invention, especially subsystems for radial damping, are applicable to levitation systems for both types.
2. Description of the Prior Art
Magnetic levitation of an object may be used as a novelty to stimulate intellectual curiosity, to isolate a levitated object from structurally coupled vibrations, or to eliminate the necessity for support bearings in rotating machinery (thus permitting higher speeds and eliminating bearing drag), to name a few examples.
Magnetic levitation of an object in a gravitational field is possible only if a magnetically generated lifting force is applied to that object to precisely counteract the force of gravity on the object. Additional forces must be applied to the levitated object to oppose any deviation in its position from the stable levitation point in space. Forces must also be applied to oppose any velocity of motion away from the stable levitation point.
There are two basic types of levitation systems: (1) those based on the axial attraction force between two magnetic elements, and (2) those based on the axial repulsion force between two magnetic elements.
In attraction type levitation systems, a stationary magnetic field generating element is positioned above a levitated member. The levitated member contains a magnetic field responsive element. There exists an attraction force between the stationary magnetic field and the levitated member. To assure long term axial position stability of the levitated member, the axial component of this attraction force must decrease with any increase in the height of the levitated member. To assure long term translational position stability of the levitated member, the horizontal components of this attraction force must oppose any errors in the translational position of the levitated member.
In attraction type levitation systems, the stationary magnetic field cannot be generated solely by a passive system of permanent magnets since permanent magnets cannot generate the magnetic field gradients required to assure decreasing lift force with increasing height. If permanent magnets were used to generate the stationary magnetic field, the levitated member would experience vertical overcenter position failure. The axial spring rate between the stationary and levitated magnetic elements would be negative so the lift force would increase with the height of the levitated member. If the magnetic lift force exceeded the weight of the levitated member even slightly, the levitated member's position would snap upward. If the magnetic lift force were even slightly less the the levitated weight, the levitated member would fall. This vertical instability can be prevented by replacing or augmenting the permanent magnet in a stationary magnetic field generating element with an electromagnet and by using an active servo control system to vary the stationary magnetic field strength as a function of the height of the levitated member. Attraction type levitation systems can be inherently stable for the long term translational position of the levitated member without the need for any active horizontal servo control system.
In an attraction type magnetic levitation system, wherein a levitated member is suspended in a gravitational field free from any visible means of support, a variable magnetic field must be generated by a stationary element and the levitated member must contain a magnetic field responsive element. The stationary magnetic field can be produced by an electromagnet or a combination of a permanent magnet and an electromagnet. (Using a permanent magnet in addition to an electromagnet has the advantage of reducing the power consumption of the electromagnet.) The magnetic field responsive element can be either a permanent magnet or a ferromagnetic material (either being capable of producing a lift force that varies with the strength of the stationary magnetic field). Using a permanent magnet rather than a ferromagnetic material has the advantage of minimizing the magnetic field strength required from the stationary field generating element and allows increased spacing between the stationary and levitated magnetic elements.
In repulsion type levitation systems, a stationary magnetic field generating element is positioned below a levitated member. The levitated member contains a magnetic field responsive element including a permanent magnet. There exists a repulsion force between the stationary magnetic field and the levitated member. To assure long term axial position stability of the levitated member, the axial component of this repulsion force must decrease with any increase in the height of the levitated member. To assure long term translational postion stability of the levitated member, the translational component of this repulsion force must oppose any errors of translational postion. In repulsion type levitation systems, the stationary magnetic field cannot be generated solely by a passive system of permanent magnets, since permanent magnets cannot generate the magnetic field gradients required to assure the generation of forces that oppose any errors in translational position. If permanent magnets were used to generate the stationary magnetic field, the levitated member would experience horizontal overcenter position failure. The translational spring rate between the stationary and levitated magnetic elements would be negative. Therefore, there would be translational forces directed away from the stable levitation point in space if there were any translational position errors, and these forces would increase with the magnitude of the position errors. This horizontal instability can be prevented by augmenting the vertical force generating permanent magnet in the stationary magnetic field generating element with a horizontal force generating electromagnet and by using an active servo control system to intentionally distort the horizontal symmetry of the stationary magnetic field as a function of the translational position error of the levitated member. Repulsion type levitation systems are inherently stable in the long term axial position of the levitated member without the need for an active axial position servo system.
In a repulsion type magnetic levitation system, wherein a levitated member is suspended in a gravitational field free from any visible means of support, a magnetic field with controllable axial nonsymmetry must be generated by a stationary element and the levitated member must contain a magnetic field responsive element. The stationary magnetic field can be produced by an electromagnet array or a combination of a permanent magnet and an electromagnet array. The magnetic field responsive element must be a permanent magnet or a permanent magnet array.
Since in all practical systems of the attraction type for generating the supporting magnetic field, the gradient of the magnetic field is far greater than the gradient of the gravitational field, in a static support system there is only one position at which the magnetic levitating force precisely counterbalances the gravitational force so that the net force on the levitated object is zero. In a practical system, however, it is impossible to develop the desired levitation at the zero net force position, since the slightest perturbation will shift the levitated object from that position and then the object will follow the force gradient in the direction of net applied force.
A practical magnetic force levitation system therefore requires a dynamic system with some sort of servomechanism feedback which is capable of adjusting the magnetic force field with a response which is quicker than the actual motion of the levitated object in response to the net force field. Such a dynamic levitation system can be achieved fairly readily by the use of one or more electromagnetic field coils to develop the magnetic levitating field and a sensor positioned to monitor the actual position of the levitated member and coupled to control the level of current applied to the electromagnetic field generating elements with sufficient speed to compensate for any perturbations or minor changes in the instantaneous position of the levitated member.
Magnetic levitation systems of this type are disclosed in U.S. Pat. Nos. 3,512,852 of North and 3,243,238 of Lyman.
In levitation systems of the type thus far described, substantial power is required to develop the electromagnetic field. Of substantial interest, therefore, is the development of arrangements, generally including combinations of permanent magnets together with the electromagnetic field generating apparatus, which do not depend on the continuous application of the electromagnetic field to support the levitated object.
The power consumption required for establishing magnetic levitation can be substantially reduced by the inclusion of suitable combinations of permanent magnets in the levitation system. The only power required in such systems is that which is needed to restore a position of balance between the permanent magnet force and the force of gravity. Arrangements of this type are disclosed in U.S. Pat. Nos. 3,860,300 of Lyman, 3,937,148 of Simpson and 4,088,379 of Perper. Another patent of Perper, U.S. Pat. No. 3,791,704, discloses apparatus for trimming the magnetic suspension system by mechanically moving adjustable permanent magnets or by varying the field of adjustable permanent magnets using at least one magnetizing winding.
The use of magnetic suspension for the rotor of a centrifuge is disclosed in an article by J. W. Beams entitled "Magnetic-Suspension Ultracentrifuge Circuits", ELECTRONICS, March, 1954, pp. 152-155. The article describes rotation in a vacuum at 20,000 revolutions per second and discusses damping arrangements including the use of a dashpot and magnetic element combination for horizontal damping of the rotor.